We wouldn't know what good is unless we know evil.
Edit: Also angels have free will as well. Satan chose to be rebellious against God. 1/3 of the angels fell with Satan. The angels that are currently in heaven probably still have a choice but they know the consequence of rebelling against the Lord.
The Lord gave us the law so we would know what good is. .. But I guess the answer is just free will..God gives each one of His creations a choice.